In order to understand the u-polarization (perpendicular) used in the main body of this paper to excite the nanoantenna, a trapezoidal nanoantenna is energized at the vertex here. This allows us to identify the fundamental radiation mechanism and the polarization of the radiated field. Due to reciprocity, this polarization is the correct polarization to excite the nanostructure in its reception mode.
The sketch of the nanoantenna considered in this supporting material is shown on the left-hand side of Figure S1a. The dimensions and Drude metal are identical to Figure 1 of the main body with a = 0.49 and identical precautions have been taken in the simulation for accurate results. The radiation efficiency of this nanoantenna when excited at the vertex has the periodic peaks characteristic of the corrugations as it is shown on the right-hand side of Figure S1a. The absolute radiation pattern associated to each peak is plotted on the left-hand side of Figure S1b. These radiation patterns indicate that the main direction of radiation is x’. In order to highlight the polarization of the radiated field, the horizontal (black curves) and vertical component (red curves) is plotted for the two orthogonal planes x’z’ and x’y’ displayed in the middle and on the right-hand side of the Figure S1b, respectively. By inspection of these orthogonal planes, we notice that the electric field is mainly polarized horizontally (u-polarized according to the main body of this paper) along the preferable direction of propagation x’, with the 2
exception of the peak at the longest resonant wavelength. Hence, the peak at longer wavelengths in the main body of this paper does not correspond to this peak, but with a different charge distribution leading to a horizontally-polarized local dipole. However, the rest peaks are indeed those associated to the fundamental operation of the nanoantenna.
To complement the charge density displayed in Figure 2b of the main body of this paper, we plot in Figure S2 the amplitude of the static charge density along with a snapshot of the electric field distribution and induced current at the middle cross section of the nanoanntena. Notice that the direction of electric field and induced current verify the multi-dipolar scenario explained in the paper.
Finally, to support that our approach leads to a broadband single hot spot, we plot in Figure S3 the vw view of the field enhancement at the cross-sectional plane of the nanoantenna corresponding to each resonant peak in the extinction cross section. Recall that Figure 2b of the main body of this paper shows the perspective view of the field enhancement, which may hide some features, and Figure 3 displays only the field enhancement at the center of the nanoantenna. Inspection of these color maps indicates that we have indeed a multi-band single hot spot that is the center of the gap. Notice that for all resonances, although one can observe several wavelength-dependent hot spots, the highest field enhancement is always achieved at the center of the nanoantenna, underlying that this hottest spot is wavelength-independent.

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